International society is divided into sovereign states and does not have an international legislature, executive, or judiciary. For this reason there is a virtual consensus of academic opinion that general customary international law is, and will remain, the main source of the law among states. This is because the concept of custom reflects the basic idea that the law that governs states is what grows out of their practice with one another, provided that they accept the practice as legally binding. In the absence of a world state, it is the actual dealings of states with one another that give rise to law, just as in a primitive community before a national state is established. The very informality of this practice, and especially the formation of the law as a product of retroactive reflection on the practice, makes general customary law a rather obscure, as well as a contested, subject. However, the passion with which international lawyers contest, exchange, and compromise views on custom attests to the importance of understanding how, in the past and in the present, the dynamic interactions of states have been seen to give rise to general customary law binding upon them.
For such a deeply contested concept as customary international law, a general overview must place most of the usually discussed elements of the topic in some relation to one another. These elements include the theoretical, historical origins of the concept, the interpretation of the terms of article 38 (1)(b) of the World Court Statute, the “two element” theory in the definition (see The Subjective Element of Custom and The Material Element of Custom), the material evidences of custom, the relation of custom to codification, treaties as potential generators of custom, the law-making potential of international organizations, the question of the binding character of custom for new states or persistent objectors, and the question of the continued relevance of customary international law in light of the speed and complexity of modern global society. The following texts provide a link between most or all of these elements. By far the most comprehensive treatment of the mainstream view of the subject is afforded in Wolfke 1993, a monograph that combines a comprehensive review of World Court practice with a strong historical understanding and careful treatment of academic doctrine. A codification of the main elements of the subject in the form of legislative propositions is provided in the Final Report of the Committee on Formation of Customary (General) International Law (International Law Association 2000). This report carries the authoritativeness of a collective and representative undertaking. Another comprehensive mainstream view of custom through World Court practice and doctrine, within a wider monograph covering all the other sources of international law, is provided in Degan 1997. A comprehensive monograph on the nature of custom in the context of its dialectical interaction with the process of codification and treaty-making generally is Villiger 1997. A systematic treatment of customary law formation within an international relations regime theory of rule formation is provided in Byers 1999. A comprehensive attempt to reconfigure the classical view of custom arising out of the historical and positivist schools of international law into a compromise of state claims and expectations is afforded in D’Amato 1971. A deconstruction of general custom, particularly as developed by the World Court, is offered in Chigara 2001, which deplores the Court’s lack of transparency and its subservience to elements of unequal power.
Byers, Michael. Custom, Power and the Power of Rules: International Relations and Customary International Law. Cambridge, UK: Cambridge University Press, 1999.
Byers translates the language of the formation of custom into international relations regime theory; explains especially the importance of leadership of great powers in creating the law and illustrates his argument with examples such as jurisdiction and diplomatic protection; straddles international law and relations.
Chigara, Ben. Legitimacy Deficit in Custom: A Deconstructionist Critique. Burlington, VT: Ashgate/Dartmouth, 2001.
Introduces the place of custom in international law, reviews theories of deconstruction and applies these theories, especially of Jacques Derrida, to a close review of some World Court cases, highlighting the lack of transparency in the Court’s reasoning and its likely subordination to the interests of great powers.
D’Amato, Antony. The Concept of Custom in International Law. Ithaca, NY: Cornell University Press, 1971.
The first to argue that customary law is not a matter of observing state practice becoming law by virtue of legal conviction of states; instead, it is a process of states arguing for, and making claims about, specific standards of behavior.
Degan, Vladimir D. Sources of International Law. The Hague: Martinus Nijhoff, 1997.
Offers a wider perspective of custom within the general system of sources of law, although that perspective is influenced by the view that the primary source of law is the will of the state (see Legal Positivism).
International Law Association. Final Report of the Committee on Formation of Customary (General) International Law. London: International Law Association, 2000.
The report is the product of an international team who had some authority to state what is a consensus of majority academic opinion in regard to custom formation. Argues that the International Court fails to resolve difficulties in the definition of custom and tries to overcome this deficiency.The report was issued at the sixty-ninth conference of the International Law Association.
Villiger, Mark E. Customary International Law and Treaties: A Manual on the Theory and Practice of the Interrelation of Sources. 2d ed. The Hague: Kluwer Law International, 1997.
Offers a picture of modern customary law formation as a continuing dialectical interaction among custom, the ongoing codification of international law, and the conclusion of multilateral, law-making treaties. Written by a legal practitioner, intended as an accessible source of reference for both academics and practitioners.
Wolfke, Karol. Custom in Present International Law. 2d rev. ed. Dordrecht, The Netherlands: Martinus Nijhoff, 1993.
Combines a deep knowledge of the case law with a sound knowledge of academic opinion; understands recent history of the development of both and presents the most salient features; easily accessible to all levels of interest.
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